Salicylic Acid promotes sloughing of dead skin cells and reduces scaling. Sometimes it’s combined with other medications, such as topical corticosteroids or coal tar, to increase its effectiveness. It may sound familiar to people how have dealt with acne medications. Salicylic acid will cause more flaking in the short term, but it has a long-term positive effect on symptoms.
Coal tar a thick, black byproduct of the manufacture of petroleum products and coal, coal tar is probably the oldest treatment for psoriasis. It reduces scaling, itching and inflammation. The tar slows the growth of the skin cells and soothes the irritation and scaling. Higher concentrations of tar in a shampoo mean the product is more potent. The Food and Drug Administration (FDA) allows shampoos with up to 5 percent tar to be sold over the counter. Coal tar is a tar so it can be messy, staining and have an odor.
Excerpts taken from: article 1 or article 2
There are many ways to apply coal tar or salicylic acids. There are creams, shampoos, lotions, ointments and gels. Which one to use, right? That depends on what it needs to do. Each one moisturizes and a plain moisturizer by itself can lessen psoriasis. It can combat and reduce the symptoms like itching, redness and scaling but without a coal tar or salicylic acid included it can only get so far.
Ointments are 80% oil and 20% water. They are ‘occlusive,’ which means they trap moisture and heat in very well. Ointments promote medication absorption over all other formulations. If an ingredient is in an ointment, it is always more potent than the exact same ingredient packaged in a cream or lotion.
Lotions are thinner than creams, and are often packaged in a pump. They absorb very quickly and feel very light on the skin. They are easier to distribute on hairy areas. Most over-the-counter body moisturizers are lotions.
Gels are emulsions that contain oil-in-water. They usually have an alcohol base. They dry into a thin, greaseless, nonstaining film. Like lotions and foams, they are ideal for spreading on hairy areas and large areas.
Creams are a semi-solid emulsion, half oil and half water. This formulation is typically the easiest to use. They spread easily, absorb quickly and wash off with water. They have a medium viscosity or heaviness, and are reasonably hydrating without feeling too heavy on your skin.